Grain Boundary Character Research Articles

Automated determination of grain boundary energy and potential-dependence using the OpenKIM framework

We present a systematic methodology, built within the Open Knowledgebase of Interatomic Models (OpenKIM) framework (https://openkim.org), for quantifying properties of grain boundaries (GBs) for arbitrary interatomic potentials (IPs), GB character, and lattice structure and species. The framework currently generates results for symmetric tilt GBs in cubic materials, but can be readily extended to other types of boundaries. In this paper, GB energy data are presented that were generated automatically for Al, Ni, Cu, Fe, and Mo with 225 IPs; the system is installed on openkim.org and will continue to generate results for all new IPs uploaded to OpenKIM. The results from the atomistic calculations are compared to the lattice matching model, which is a semi-analytic geometric model for approximating GB energy. It is determined that the energy predicted by all IPs (that are stable for the given boundary type) correlate closely with the energy from the model, up to a multiplicative factor. It thus is concluded that the qualitative form of the GB energy versus tilt angle is dominated more by geometry than the choice of IP, but that the IP can strongly affect the energy level. The spread in GB energy predictions across the ensemble of IPs in OpenKIM provides a measure of uncertainty for GB energy predictions by classical IPs.

Nanowire single-crystal grain and single grain boundary silicon field effect transistors for direct electrical characterization of grain boundaries

Single crystal grain (s-G) and single grain boundary (s-GB) Si field effect transistors (FETs) were investigated for direct measurement of the electrical activity of GBs. Ultrahigh-performance nanowire s-G FET with 1360 cm2V−1s−1 electron field-effect mobility (μ ef) was achieved on a transparent substrate. The significant difference of μ ef values between nanowire and microwire FETs proved that uniaxial tensile strain typically contributed to μ ef enhancement in nanowire FETs. Longitudinal GBs typically increased the leakage current and S factor due to a large donor concentration. The donor concentration generated by Σ3 and other CSL-type s-GBs was estimated.

Influence of grain boundary energy anisotropy on the evolution of grain boundary network structure during 3D anisotropic grain growth

In this paper, we present the results of grain growth simulations in three-dimensions using an existing level set method. Most of the previous grain growth studies have been either isotropic or, if they are not isotropic, consider simplified models of anisotropy. We perform these simulations using three different grain boundary (GB) energy functions (a realistic 5D GB energy function, a Read–Shockley energy model that depends only on disorientation angle, and an isotropic GB energy model). We compare the results in terms of several statistical microstructural descriptors (crystallographic texture, GB character distribution, triple junction distribution (TJD), etc.). In addition to considering different energy models, we also compare the evolution of microstructures that have different initial crystallographic textures (fiber texture and random texture). We find that the morphological evolution of individual grains can be completely different depending on the GB energy function employed. However, we also find that certain spatially independent microstructural statistics (e.g. orientation distribution function, misorientation distribution function, and TJD) are similar at steady state for all of the tested GB energy functions.

Defeating hydrogen-induced grain-boundary embrittlement via triggering unusual interfacial segregation in FeCrCoNi-type high-entropy alloys

Metallic materials are mostly susceptible to hydrogen embrittlement (HE), which severely deteriorates their mechanical properties and causes catastrophic failures with poor ductility. In this study, we found that such a long-standing HE problem can be effectively eliminated in the Fex(CrCoNi)1-x face-centered-cubic (fcc) high-entropy alloys (HEAs) by triggering the localized segregation of Cr at grain boundaries (GBs). It was revealed that increasing the Fe concentration from 2.5 to 25 at. % leads to substantially improved HE resistance, i.e., the ductility loss decreases from 70% to 6%. Meanwhile, the fracture mode transformed from the intergranular to the transgranular mode. Multiscale microstructural analyses demonstrated that the Fe2.5Cr32.5Co32.5Ni32.5 and Fe25Cr25Co25Ni25 alloys show negligible differences in the phase structure, grain size, and grain-boundary (GB) character. However, interestingly, the near atomic-resolution elemental mapping revealed that an increased Fe concentration promotes the nanoscale Cr segregation at the GBs, which is primarily motivated by the strong repulsive force between Cr and Fe and the low self-binding energy of Cr. Such unusual interfacial segregation of Cr, which has not been reported before in the Fe25Cr25Co25Ni25 alloy, helps enhance the GBs’ cohesive strength and suppresses the local hydrogen segregation at GBs due to the deceased GB energy, leading to the outstanding HE resistance. These findings decipher the origins of the vastly-improved HE resistance in current FeCrCoNi-type HEAs, and meanwhile, provide new insight into the future development of novel high-performance structural alloys with extraordinary immunity to hydrogen-induced damages.

Structure characterization of interior invisible single grain boundary by nanorobot pick-and-place grain in bulk ZnO under SEM

• Invisible grain boundary is first creatively characterized using nanorobot in SEM. • Irregular shape of invisible grain boundary can be exhibited in plane X-Y. • Three-dimensional virtual reconstruction is utilized to verify irregular structure. In order to independently characterize invisible structure, a new approach is first creatively proposed to express interior invisible single grain boundary using nanorobot pick-and-place single grain in SEM. Picking of a single grain handled by single probe and then rolling-over placing manipulated by dual-probes can be sequentially conducted by nanorobot. The upper and lower invisible surface interface layer profile of single grain can be extracted by SEM imaging in plane X-Y. Experimental results demonstrate that interior invisible single grain boundary characterized by nanorobot seems to be reasonably feasible. The upper and lower surface structures of single grain in plane X-Y can also be virtually modeled by 3D software. This method can further provide a unique characterization approach of interior invisible structure of single grain boundary.

Microstructure Evolution of AZ91 Magnesium Alloy Welded Joint under Magnetic Field and NiCl2 Activated Flux

As the lightest engineering materials, magnesium alloys have been widely used. Because of the specific chemical and physical characteristics, the weldability of magnesium alloy is poor. Adopting suitable welding technology and improving the quality of magnesium alloy welded joints is key to their successful application. According to previous research data, it was found that the combined action of magnetic field and activated flux has a positive effect on improving-welding efficiency and improving the properties of a welded joint, butanalysis of microstructure evolution is insufficient. In this paper, AZ91 magnesium alloy was welded by TIG welding with activated flux and external longitudinal AC magnetic field. The phase composition and microstructure evolution were investigated. The experimental results revealed that the phase composition of welded joint was not changed due to the introduction of the magnetic field and activated flux, the growth patterns of grain in the weld seam and heat-affected zone were different. When the activated flux amount was 3 mg/cm2 with the effect of the magnetic field, the grain size of the weld seam was the finest, which was 18.96 μm. However, the grain size of the weld seam was larger than that of base metal. The crystallographic characteristics of grain boundaries in the weld seam and base metal were both LAGBs. The microstructure of the weld seam was messier than the base metal due to the larger misorientation angle. Under the combined action of the magnetic field and activated flux, the crystallization nucleation condition of the molten pool was changed, the formation of twins was promoted, and the crystal could selectively grow parallel with the (0001) basal plane.

In-situ mapping characterization of structure and electrical property of grain boundary in polycrystalline ZnO using nanorobot in SEM

Different from the existing methods of individual characterization of structure or electrical property, a unique approach for in-situ mapping characterization of single grain boundary using nanorobot in SEM is proposed to simultaneously realize the characterization of structure and electrical property in micro-area polycrystalline ZnO. Z-direction height differences constructed by multiple adjacent grains relative to a single target grain can be individually probed by the nanorobot single end tip and correspondingly virtually reconstructed. Electrical impedances complex planes of grain boundaries in-situ detected by nanorobot dual probes can be fitted by equivalent circuits. Experimental results illustrate that in-situ mapping characterization of structure and electrical property of single grain boundary using nanorobot in SEM can be verified. A single grain boundary layer, composed of adjacent multi-grains, seems to be nonlinear and unequal height along the Z direction. Z-direction relative height differences probed by the nanorobot can actually be regarded as the Z-direction projected lengths of surrounding grain boundary interfaces of single target grain. A three dimensional stereogram of single target grain boundary can be virtually reconstructed based on 3D software. It is deduced that schottky barrier existing in grain boundary interface exhibits larger height difference, basically proportional to the thickness of grain boundary in plane X–Y. Furtherly, it can promote an advance in in-situ mapping characterization of structure-property of single/multi micro area in other bulk inorganic polycrystalline bulk materials.

A Correlative Study of Interfacial Segregation in a Cu-Doped TiNiSn Thermoelectric half-Heusler Alloy.

The performance of thermoelectric materials depends on both their atomic-scale chemistry and the nature of microstructural details such as grain boundaries and inclusions. Here, the elemental distribution throughout a TiNiCu0.1Sn thermoelectric material has been examined in a correlative study deploying atom-probe tomography (APT) and electron microscopies and spectroscopies. Elemental mapping and electron diffraction reveal two distinct types of grain boundary that are either topologically rough and meandering in profile or more regular and geometric. Transmission electron microscopy studies indicate that the Cu dopant segregates at both grain boundary types, attributed to extrusion from the bulk during hot-pressing. The geometric boundaries are found to have a degree of crystallographic coherence between neighboring grains; the rough boundaries are decorated with oxide impurity precipitates. APT was used to study the three-dimensional character of rough grain boundaries and reveals that Cu is present as discrete, elongated nanoprecipitates cosegregating alongside larger substoichiometric titanium oxide precipitates. Away from the grain boundary, the alloy microstructure is relatively homogeneous, and the atom-probe results suggest a statistical and uniform distribution of Cu with no evidence for segregation within grains. The extrusion suggests a solubility limit for Cu in the bulk material, with the potential to influence carrier and phonon transport properties across grain boundaries. These results underline the importance of fully understanding localized variations in chemistry that influence the functionality of materials, particularly at grain boundaries.

Model of void nucleation on grain boundaries in dynamic failure of ductile metals incorporating interface incompatibility

Void nucleation on grain boundary (GB) has been regarded as an important mechanism of damage initiation in ductile polycrystalline materials under dynamic loading. The high tensile stress induced by this loading mode enables interface incompatibility (i.e. the incompatibility of mechanical properties across the GB) to significantly affect intergranular spall damage initiation. In the present work, the concept of compatibility energy release rate is proposed to quantify the influence of interface incompatibility on interfacial failure. A tensor-formed criterion incorporating the compatibility energy release rate is established for modeling GB void nucleation in dynamic failure of ductile metals. We show that considering interface incompatibility enables the model to accurately describe the dependence of GB void nucleation on multiple factors reported by experiments and simulations, including the loading state and the GB characteristics. A statistical model on the basis of the criterion, aimed at further analyzing statistical features of GB void nucleation, provides a valid reference for quantitatively assessing the GB damage resistance and gives a physical explanation for the widely-adopted Weibull-form distribution of the nucleation stress.

Coupling effect of torsion and electric pulse treatment on grain boundary regulation and plasticizing of nickel wire

Thermomechanical process (TMP) for grain boundary (GB) regulation is conducted on pure nickel wires to study the coupling effect of torsion and electric pulse treatment (EPT) on GB characteristics, recrystallization behavior and plasticity. The results show that high fraction of SBs, refined grains and high elongation can be achieved. The increase of most EPT parameters is able to increase the special boundary fraction and grain size. With the rising of current density, the elongation of samples with different torsional cycles tends to be uniform. Both microstructure characterization and theoretical calculation manifest that only Joule thermal effect is insufficient to stimulate recrystallization while athermal effect contributes more in accelerating recrystallization and refining grains. The electron wind force enhances the stored energy difference among deformed grains and thus reduces the critical nucleation radius, which evidently improves the nucleation rate. The interfaces between nuclei and matrixes are mostly Σ3 boundaries. Subsequently, the rapid decline of driving force suppresses nuclei growth and eventually the material retains fine grains and a high fraction of special boundaries. Σ3 boundaries are conducive to material plasticizing because they can relieve nearby stress concentration by transforming into random boundaries during plastic deformation. Dislocation pile-up is much slighter on Σ3 boundaries than random boundaries.

Correction to: Structural and crystallographic characterization of grain boundaries coarse particles in an Al–Mg-Si alloy, using convergent beam electron diffraction

Correction to: Structural and crystallographic characterization of grain boundaries coarse particles in an Al–Mg-Si alloy, using convergent beam electron diffraction

Statistical analysis of misorientation to goss orientation of highly grain-oriented silicon steel

Grain-oriented (GO) steel is primarily used as a core material for transformers. For better magnetic performance, the high selectivity to the exact {110} 〈001〉 orientation has been a target for GO steel. Commercially available high-grade of GO steel consists of grains with a misorientation <3° from the {110} 〈001〉 orientation. However, the magnetic properties of the commercial GO steel implies that the GO steel should contain grains with larger deviation angles. As GO steel has much larger grains (diameters of several centimeters) than low-carbon steel, the conventional microscopy method used to measure the orientations of the grains is not suitable for GO steel. In this study, Laue diffraction was used to analyze the orientation of large grains of HI-B GO (HGO) steel to obtain statistically reliable topological data and characterize the grain boundaries between abnormally grown grains. The measured area was 6300 cm2, which contained 1230 grains. The analyzed HGO steel sheet had 1/3 of the grains with orientations close to an exact Goss orientation, which was defined as a range of orientations with a misorientation angle <5°, and 2/3 of the grains with higher misorientation to the Goss orientations. The topological and grain-boundary characteristics of the grains based on the misorientation to the exact Goss orientation are discussed in detail.

A comprehensive analysis of ultrasonic pulse current reducing hot cracking in Inconel 718 welds

A novel self-induced ultrasonic tungsten inert gas (U-TIG) welding has been successfully applied to weld the Inconel 718 sheet with thickness of 2 mm to reduce hot cracking. The welding arc, weld formation, microstructure and micro-hardness distribution between TIG and U-TIG welding were analyzed systematically. The relationship between micro-hardness and characteristics of grain size, grain orientation, grain boundary, segregation and recrystallization proportion in transition zone (from weld metal zone to heat affected zone) was investigated. The characteristics of sizes, disorientations and boundaries of adjacent grains, as well as the micro-strain distributions in weld metal zones of TIG and U-TIG welds were analyzed contrastively. The results indicate that the welding arc with symmetrical shrinkage induced by ultrasonic pulse current greatly intensifies the central depression of the weld bead surface, which benefits the release of welding thermal stress. With the promoted transformation of coarse columnar dendrites to fine equiaxed dendrites with similar orientation in U-TIG welds, the segregation in grain boundaries is greatly reduced, which reduces potential of the hot cracking initiation and propagation. In addition, the improvement of micro-hardness distribution is contributed by grain refinement and segregation dispersion in U-TIG welds.

Atomic energy in grain boundaries studied by machine learning

Grain boundaries (GBs) have been studied for decades, but it remains a challenging task to describe characteristic GB properties by simple structural descriptors, especially at the local atomic level. In this paper, we use the atomic descriptor based on the smooth overlap of atomic positions (SOAP) to study the atomic energy at GBs by using machine learning and propose a route to simplify it. It is found that, compared with conventional local atomic descriptors such as the Voronoi index, excess volume, centrosymmetry, or local entropy, the SOAP vector shows excellent predictive performance for the atomic energy among the 172 Al and 388 Ni coincidence site lattice (CSL) GBs as well as general GBs in the nanocrystalline model. Additionally, we successfully used the datasets of GBs and amorphous models to predict the atomic energy of one another, which proves the similarity between the local atomic environments (LAEs) in GBs and the amorphous state. Furthermore, the distribution of local distortion factors based on the SOAP vector shows the transition in atomic pack ordering from special CSL GBs to general GBs and the amorphous structures. The simplified descriptor we propose can reduce the original SOAP vector from &gt;1000 features to only a few yet still shows the superior predictive performance of the atomic energy at GBs in all cases than the conventional descriptors combined. It is expected that the simplification process can be adapted to study more complex GB behaviors. A simple and efficient descriptor of the LAEs should allow us to have a clearer picture of the structure-property correlation in GBs, which is essential for GB engineering.

Structural and crystallographic characterization of grain boundaries coarse particles in an Al–Mg–Si alloy, using convergent beam electron diffraction

Structural and crystallographic characterization of grain boundaries coarse particles in an Al–Mg–Si alloy, using convergent beam electron diffraction

Inhomogeneous dealloying kinetics along grain boundaries during liquid metal dealloying

In this study, the inhomogeneous dealloying phenomenon during the liquid metal dealloying (LMD) was investigated using Fe50Ni50+Mg and (FeCo)50Ni50+Mg systems. For the Fe50Ni50+Mg system, the inhomogeneous dealloying and wetting of Mg melt occurred along triple junction (TJ) and grain boundary (GB). Temperature increase enhances the inhomogeneous dealloying kinetics and leads to the formation of the plate-shaped abnormal ligaments at the GB region. The energy banlance between a GB energy (γGB) and solid-liquid interface energies (γsl) is the key factor governing the inhomogeneous dealloying and wetting. Particularly, the low-energy twin boundaries were unaffected by the inhomogeneous dealloying. Therefore, precursor microstructure is an important factor determining the final morphology of dealloyed material as well as its physical properties. In the case of the (FeCo)50Ni50 precursor, all TJ and GB were stable against the preferred penetration of Mg melt from 600 °C to 800 °C. It was concluded that a minor addition of alloying elements (V or Cr) changes GB characteristics as well as γsl of the precursor alloy. Consequently, this significantly influences dealloying mechanisms and final morphology of the dealloyed material. The current findings demonstrate the importance of GB engineering in the precursor materials for the technological application of liquid metal dealloying for the synthesis of advanced structural and functional materials.

Effects of thermomechanical processes on grain boundary characteristics and plasticization mechanism of micro nickel wires

“Tensile and annealing” grain boundary (GB) modification is carried out on micro nickel wires to investigate the influence of thermomechanical parameters on the microstructure and plasticity, and to clarify the relationship between GB evolution and plasticity. A high proportion of special boundaries (SBs), the optimized GB characteristic distribution (GBCD) and a 9% reduction in hardness can be achieved by stretching the sample by 10% and then annealing at 900 °C for 5 min. Microstructure analysis indicates that numerous SBs formed in the early stage of recrystallization. Such SBs especially Σ3 boundaries contribute the most to the plasticization of nickel wires. The grains with high Schmid factor always have 〈001〉 orientation, which is conductive to the formation of Σ3 boundaries. What's more, a microscopic indicator concerning incoherent Σ3 boundaries that can quantitatively characterize the macroscopic hardness is presented, and thus the plasticization mechanism is analyzed accordingly.

Understanding the Impacts of Grain Size Variation, Distribution, and Recombination Losses in Halide Perovskites: A Generalized Semi‐Analytical Model from Thin‐Film to Photovoltaics

Fast crystallization growth in thin‐film photovoltaic materials, often associated with solution‐based low‐cost processing, induces crystal misorientation defects called grain boundaries. Grain boundaries are generally believed to be sources of efficiency losses in photovoltaics, but their impact in halide perovskite materials is not at all trivial. Herein, a semianalytical model is proposed that leverages the standard rate equation to elucidate the microscopic impact of grain boundaries in halide perovskites. Grain boundary and surface recombination can have different impacts up to 3 orders of magnitude to carrier lifetime and quasi‐Fermi‐level splitting. Variation in grain sizes can play a greater role than the average grain size, leading to a 2% drop in photovoltaics efficiency. Heteroscedasticity in grain variation is even more problematic for large grains, which explains why samples with small grains may perform better than larger ones. A map of the power conversion efficiency of individual grains is simulated, showing that recombination current exchange among adjacent grains reduces the overall efficiency by 4%, depending on their spatial arrangement. Although carrier transport is uniquely dictated by the characteristics of grain boundaries, the generalized scheme presented herein should be useful for accommodating more complexity beyond simply the grain size effect.

Deformation twins stimulated by {11[formula omitted]2} twinning in adjacent grain in titanium

Deformation twins are likely nucleated at and impeded by grain boundaries (GBs), developing shear transformation bands. Knowing the accommodated deformation in adjacent grains to the twinning shear is essential for understanding mechanical properties and the evolution of microstructure and texture of polycrystalline aggregates. Based on EBSD analysis, we characterize and statistically group accommodated deformation modes in the neighbor to {112¯2} compression twins (CTWs) in titanium with respect to the misorientation axis/angle. Two types of CTWs are preferably activated in adjacent grains with low misorientation angles, two types of extension twins (ETWs) are favorably activated in adjacent grains with high misorientation angles, and some of {112¯2} CTWs terminate at GBs with high misorientation angles. We discuss the preference of accommodated deformation modes and the selection of twin variants based on the SDGA criterion which couples the displacement gradient accommodation (DGA) and stress driven accommodation (SDA) criteria. Although misorientation angle of GBs is widely used to predict the preferred deformation modes, we demonstrate that misorientation axis that was often ignored, must be considered in determining the preferred deformation modes especially for large misorientation angles. Finally, we predict the misorientation angle/axis pairs with respect to the activity of four twinning modes and two slip systems as the best accommodation mechanism in adjacent grains. The results are useful for the implementation of intergranular deformation mechanisms in multiscale modeling and the design of HCP materials with specific GB characteristics.

Towards the dependence of radiation damage on the grain boundary character and grain size in tungsten: A combined study of molecular statics and rate theory

Grain boundary (GB) character and radiation conditions restrict the in-depth understanding of the effectiveness of tailoring grain size in controlling the radiation damage and promoting the radiation resistance of materials. Here, we constructed 46 [0 0 1] W symmetric tilt GBs with tilt angle varying from 6.73° to 82.37° Then, molecular statics (MS) was used to investigate the impact of GB character (tilt angle, GB energy, Σ) on the defect energetics. The present results suggested that it is energetically favorable for Vs/SIAs to segregate into the GB and GBs have the biased absorption of SIAs over Vs. Some high-angle and high-energy GBs could provide larger energetic driving force for the V/SIA to segregate. Multiple Vs/SIAs tended to be bound together inside the GB, with a varying binding strength. Using steady-state rate theory (RT) calculations, we explored the Vs accumulation in the W systems with different grain sizes (50–1000 nm) under the typical radiation conditions of temperature (300–1800 K) and dose rate (10−8–102 dpa/s). In particular, based on the results of MS calculations, the effect of GB character on the Vs accumulation was examined by varying V segregation energies (0.99–2.04 eV). In the process, two extreme values (2.04 and 0.99 eV) of V segregation energy were extracted to investigate its impact, with other values being in this range. The results suggested that nanocrystalline (NC) W would exhibit better radiation tolerance than coarse-grained (CG) W under the conditions of a relatively high temperature and low dose rate. A high V segregation energy led to a low damage accumulation in the grain interior. Some high-angle and high-energy GBs with a large V segregation energy and a small V–V binding energy were expected to improve the radiation resistance of NC.